The disclosure relates to a laser unit and a non-transitory computer-readable storage medium.
With miniaturization and high integration of a semiconductor integrated circuit, improvement of resolution is demanded in a semiconductor exposure apparatus. Hereinafter, the semiconductor exposure apparatus is simply referred to as “exposure apparatus”. Shortening of a wavelength of light outputted from an exposure light source is accordingly proceeding. A gas laser unit is used for the exposure light source, in place of an existing mercury lamp. Currently, as a gas laser unit for exposure, a KrF excimer laser unit that outputs ultraviolet rays of a wavelength of 248 nm and an ArF excimer laser unit that outputs ultraviolet rays of a wavelength of 193 nm are used.
As current exposure technology, liquid immersion exposure is practically used in which a clearance between a projection lens on exposure apparatus side and a wafer is filled with a liquid to change a refractive index of the clearance, shortening an apparent wavelength of light from the exposure light source. When the liquid immersion exposure is performed with use of the ArF excimer laser unit as the exposure light source, ultraviolet rays with a wavelength of 134 nm in water is applied to the wafer. This technology is referred to as ArF liquid immersion exposure. The ArF liquid immersion exposure is also referred to as ArF liquid immersion lithography.
Since a spectral line width in free oscillation of the KrF and ArF excimer laser units is wide and about 350 pm to about 400 pm, color aberration of laser light (ultraviolet rays) that is reduced and projected on the wafer by the projection lens on the exposure apparatus side occurs, which results in decrease in resolution. It is therefore necessary to narrow the spectral line width of the laser light outputted from the gas laser unit to an extent that the color aberration is negligible. The spectral line width is also referred to as a spectrum width. A line narrow module including a line narrowing device is thus provided in a laser resonator of the gas laser unit, which achieves narrowing of the spectrum width. Note that examples of the line narrowing device may include an etalon and a grating. The laser unit narrowed in spectrum width in this way is referred to as a line narrowing laser unit. Examples of the technology in this field may include Japanese Patent No. 3773858, U.S. Pat. No. 6,973,111, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2010-519782, U.S. Pat. No. 7,835,414, U.S. Patent Application Publication No. 2013/0100980 specification, and U.S. Pat. No. 6,735,236.